Vectors were constructed which contain promoterless genes for chloramphenicol (cam) or tetracycline (tet) resistance, as promoter-probe plasmids. Escherichia coli cells harboring these plasmids are sensitive to cam or tet but resistant to ampicillin. In plasmids pKK231 -1 and pKK232 -8 the gene for cam acetyltransferase (CAT) and in pKK175 -6 the gene for tet resistance are flanked by efficient transcription terminators, preventing transcription from other pBR322 promoters into the antibiotic resistance region. In one of the vectors, pKK232 -8, translational stop codons were introduced in all three reading frames upstream from the initiation codon of the cat gene. If a DNA fragment containing a promoter is inserted into one of the cloning sites upstream from the antibiotic genes, cells carrying such plasmids acquire resistance to cam or tet. Using these vectors two restriction fragments that contain promoters were identified. One of these fragments contains sequences upstream from an unidentified gene ( ORFII ) located distal to the rrnB rRNA operon of E. coli.